Among Sinclair's feature film credits are "I Will...I Will...for Now," "Cornbread, Earl and Me" (both 1975), "Leadbelly" (1976), "Star Trek IV: The Voyage Home" (1986) and "Coming to America" (1988). Her final film role was the voice of the Lion Queen in Disney's "The Lion King" (1994).

A capable and versatile businesswoman as well, Sinclair was an art dealer, chairwoman of the women's clothing manufacturer, Madge Walters Sinclair Inc., and owner of the Action Income Tax Service.